ASC7621 Ver la hoja de datos (PDF) - Unspecified
Número de pieza
componentes Descripción
Fabricante
ASC7621 Datasheet PDF : 50 Pages
| |||
aSC7621
Series Resistance
Any external series resistance in the connections from
the aSC7621 to the CPU pins should be accounted for in
interpreting the results of a measurement.
The impact of series resistance on the measured
temperature is a result of measurement currents
developing offset voltages that add to the diode voltage.
This is relatively constant with temperature and may be
corrected with a fixed value in the offset register. To
determine the temperature impact of resistance is as
follows:
where:
ΔTR = RS × ΔID / TV
or ,
ΔTR
=
RS
×
90μ A
230μV / °C
=
RS
×
0.391°C
/
Ω
ΔTR = difference in the temperature reading from actual.
RS = total series resistance of interconnect (both leads).
ΔID = difference in the two diode current levels (90µA).
TV = scale of temperature vs. VBE (230µV/°C).
For example, a total series resistance of 10Ω would give
an offset of +3.9°C.
Discrete Remote Diodes
transistor’s die temperature is usually not the temperature
of interest and care must be taken to minimize the
thermal resistance and physical distance between that
temperature and the remote diode. The offset and
response time will need to be characterized by the user.
Board Layout Considerations
The distance between the remote sensor and the
aSC7621 should be minimized. All wiring should be
defended from high frequency noise sources and a
balanced differential layout maintained on Remote + and
Remote -.
Any noise, both common-mode and differential, induced
in the remote diode interconnect may result in an offset in
the temperature reported. Circuit board layout should
follow the recommendation of Figure 15. Basically, use
10-mil lines and spaces with grounds on each side of the
differential pair. Closer spacing may also be used if
required by layout, but the priority is balance of diode
path and minimum vias. Choose the ground plane closest
to the CPU when using the CPU’s remote diode.
10 mil
Line
10 mil
Space
GND
Remote +
Remote -
GND
When sensing temperatures other than the CPU or GPU
substrate, an NPN or PNP transistor may be used. Most
commonly used are the 2N3904 and 2N3906. These
have characteristics similar to the CPU substrate diode
with non-ideality around 1.0046. They are connected with
base to collector shorted as shown in Figure 14.
While it is important to minimize the distance to the
remote diode to reduce high-frequency noise pickup, they
may be located many feet away with proper shielding.
Shielded, twisted-pair cable is recommended, with the
shield connected only at the aSC7621 end as close as
possible to the ground pin of the device.
2N3906
Remote +
aSC7621
Remote -
2N3904
Remote +
aSC7621
Remote -
Figure 14 Discrete Remote Diode Connection
As with the CPU substrate diode, the temperature
reported will be subject to the same errors due to non-
ideality variation and series resistance. However, the
Figure 15 Recommended Remote Diode
Circuit Board Interconnect
Noise filtering is accomplished by using a bypass
capacitor placed as close as possible to the two pairs of
aSC7621 Remote + and Remote - pins. A 1.0nF ceramic
capacitor is recommended, but up to 3.3nF may be used.
Additional filtering takes place within the aSC7621.
It is recommended that the following guidelines be used
to minimize noise and achieve highest accuracy:
1. Place a 0.1µF bypass capacitor to digital ground
as close as possible to the power pin of the
aSC7621.
2. Match the trace routing of the Remote + and
Remote - leads and use a 1.0nF filter capacitor
close to the aSC7621. Use ground runs along side
the pair to minimize differential coupling as in
Figure 15.
3. Place the aSC7621 as close to the CPU or GPU
remote diode leads as possible to minimize noise
and series resistance.
4. Avoid running diode connections close to or in
parallel with high-speed busses or 12V, staying at
least 2cm away.
© Andigilog, Inc. 2006
- 46 -
www.andigilog.com
October 2006 - 70A06010
Share Link: 